1. Field of the Invention
This invention relates to an integrated optical modulator for producing a duobinary modulated optical signal, and also to a modulator system using such a modulator. The invention further relates to a modulating method, for producing a duobinary modulated optical signal.
2. Description of the Prior Art
The ever increasing demand for telecommunication systems has led to the development of optical high bandwidth transmission systems, having multi-gigabit data rates. As the bandwidth of a telecommunication system increases, previously unimportant system factors are becoming critical to the successful operation of the system.
One factor which has become critical is the chromatic dispersion of an optical data signal, where the broadening of the digital pulses as the signal is propagated through the transmission system degrades the system performance and causes increased bit-error rates. Since chromatic dispersion is essentially a function of the propagation velocity of light in a fibre and the wavelength of that light, signals with a broader optical spectrum will be more severely corrupted by inter-symbol interference effects than signals with a narrower spectral content. The broadening of the optical spectral content of a given signal is usually referred to as the "chirp" of the signal.
The use of dc biased narrow linewidth laser sources in conjunction with zero-chirp external modulators has resulted in the information bandwidth itself being the limiting and controlling factor of the spectral content of a signal, and hence the severity of the inter-symbol interference at a receiver. If all other parameters remain constant for such a system, a reduction in transmission bandwidth should result in a reduction of the chromatic dispersion penalty for the system.
The use of so-called duobinary codes is one way of reducing the information bandwidth, since a duobinary coded signal has half the transmission bandwidth of a binary signal. A duobinary coded signal is in fact a ternary signal having three levels--that is, off, half-on and on. Such a transmission system may also use some degree of controlled inter-symbol interference, to introduce correlation into the signal.
In U.S. Pat. No. 5,303,079 there is described a modulator which is configured to allow the chirp of a signal to be adjusted and controlled, but that modulator can be used to provide duobinary modulation. This is because the modulator has two electrodes, one on each arm respectively of the modulator, and which electrodes can be driven separately by a duobinary driver circuit.